1. Field of the Invention
This invention relates generally to caster assemblies for the support and movement of articles of furniture, as well as factory and baggage trucks, portable equipment and the like. In particular, the invention relates to a caster assembly which is capable of supporting heavy loads without becoming impaired and having a high resistance to shock forces.
2. Status of Prior Art
A caster assembly is composed of one or more wheels mounted on a swivel frame for the support and movement of the article such as a movable cart, trolley or gurney on which the assembly is installed. In a typical caster assembly such as one attached to the leg of a chair, a load-supporting base member secured to the base of the leg has projecting therefrom a vertical axle oh which one end of a swivel frame is pivoted. Mounted below the other end of the frame is a ground-engaging caster wheel rotatable on a horizontal axle.
When a chair which has attached to each of its legs a conventional caster assembly, each caster wheel swivels about a vertical axle projecting from the leg of the chair to assume an orientation in which the horizontal axis of rotation of the wheels is normal to the chosen direction of chair movement. Hence as the chair advances in the chosen direction, the caster wheels on the legs rotate freely.
The quick angular re-alignment of the caster wheels with the direction of movement which takes place as the direction is changed not only makes possible swift transportation of the load but also prolongs the effective life of the caster wheels.
As noted in U.S. Pat. No. 4,254,532 to Hager in which is disclosed a caster assembly, the axis of each caster wheel is necessarily offset horizontally from the swivel axis in order to cause the caster to trail properly. As a consequence, the caster-assembly is vulnerable to impact forces. Thus when the ground-engaging wheel of the assembly is traveling at a fast speed and it strikes an obstruction in its path, the resultant impact force may break the caster.
In order to reduce stresses imposed on the caster bracket or swivel frame when the caster strikes an obstruction, and to avoid injury to the caster, Hager interposes a support roller between the swivel frame and the load at a point spaced radially outwardly from the swivel axis.
A standard caster assembly is designed by its manufacturer to operate efficiently when the load it bears is no greater than a predetermined value. Hence a caster assembly is given a load rating by its manufacturer. Casters intended for a baggage truck are designed to safely support a load for which its wheels are rated. But as is often the case, when the truck in use is overloaded with baggage, the caster assembly may be incapable of surviving this excessive load.
The concern of the present invention is with the ability of a caster assembly whose wheels are substantially the same as those in a standard caster assembly to withstand loads and impact forces that the standard assembly cannot tolerate.
As noted in PCT Publication WO 01/38201, in a castor assembly in which a castor wheel is mounted on a swivel, the axle of the caster is horizontally offset with respect to the axis of the swivel. This creates a moment of force that may be of sufficient strength to break the vertical axle on which the castor swivel is pivoted.
The moment of force is the result of the force produced by a load multiplied by its perpendicular distance from a point of turning. A moment of force is equivalent to a torque which gives rise to torsion. In the context of a caster assembly, this torsion may twist and break the swivel axle and, in doing so disable the assembly.
When the load borne by a castor assembly is close to or exceeds its load rating, then with a conventional caster assembly the resultant high moment of force may be powerful enough to disable the assembly. To reduce the moment of force assembly disclosed in the above-identified PCT publication, includes at least one support roller that is interposed between the pivoted swivel on which a caster wheel is mounted and the load imposed on the swivel. While this PCT arrangement does reduce the moment of force, it does not fully cancel it out so that the caster assembly is capable of tolerating very heavy loads.
Moreover, the support roller in the PCT publication interposed between the swivel and a base member supporting the load, lengthens the overall height of the assembly so that its height is significantly greater than that of a standard caster assembly which lacks a support wheel. The ideal caster assembly is one whose swivel is inclose proximity to a base member supporting the load so as to raise the load above ground no more than necessary to accommodate the ground-engaging caster wheel.
Also of prior art interest is U.S. Pat. No. 4,509,227 to Keane which discloses a swivel caster structure in which a caster wheel is carried on a base. A roller functioning as a force transfer agent is carried by the base for transmitting an overlying load (in this case a vacuum cleaner) vertically downward toward the caster wheel away from the swivel axis in order to improve the effective life of the structure.
In view of the foregoing, the main object of this invention is to provide an improved caster assembly in which a caster which is swiveled from a vertical axle projecting below a base member supporting the load, the assembly including at least one support roller interposed between the swivel and the base member to protect the assembly from adverse effects resulting from an excessive load or impact forces.
More particularly, an object of this invention is to provide a caster assembly in which a frame on which is mounted a caster wheel is pivoted from a vertical axle, the frame has mounted thereon a pair of support rollers which are symmetrically disposed with respect to the vertical axle and to the caster wheel, the arrangement being such as to cancel out the moment of force and to enhance the resistance of the assembly to the adverse effects of an excessive load and impact forces.
A significant feature of the invention is that the overall height of the assembly is not much greater or no greater than that of a standard assembly having a like caster wheel, for in an assembly in accordance with the invention, the load-support rollers are nested in wells or recesses formed in the swivel frame so that only the uppermost segment of each roller is raised above the frame to engage the load.
Briefly stated these objects are attained in a castor assembly for supporting a load such as an article of furniture so that it can easily be moved in any desired direction. The assembly includes a load-bearing base member from which a vertical axle is projected which pivots a swivel frame. Mounted on the frame is a ground-engaging caster wheel having a horizontal axle. Also mounted on the swivel frame is at least one support roller which engages the base member to support the load imposed therein. The position of the roller relative to the caster wheel is such as to cancel out the moment of force between the vertical axle and the caster wheel, thereby protecting the assembly from the adverse effects of an excessive load and of impact forces.
In a preferred embodiment of the invention, the swivel frame is generally triangular in form and is pivoted at its apex on the vertical axle projecting below the load-bearing base member. The caster wheel is mounted below the frame adjacent its rear end midway between its corners on a central axis that extends to the pivot point. Hence when the frame is turned, the caster wheel is movable in a circular orbit whose center is the pivot point. A pair of load-supporting rollers are mounted adjacent the respective rear corners of the triangular frame on either side of its central axis and symmetrically with respect to the caster wheel. Hence, the rollers are movable in a circular orbit which is concentric with and adjacent to the main caster wheel orbit to provide a common line of action.